Slotted ring antenna



y 1957 A. ALFORD ET AL 2,798,217

' SLOTTED RING ANTENNA Filed July 28, 1955 m "2 H) k. 1

m m R m v INVENTOR. By zzzszfmzx United States Patentfihice 2,798,217 Patented July 2, 1957 SLOTTED RING ANTENNA Andrew Alford and Harold H. Leach, Winchester, Mass; said Leach assignor to said Alford Application July 28, 1955, Serial No. 524,946 14 Claims. (Cl. 343-470) This invention relates to antennas for ultrahigh frequencies and more particularly to antennas for broadcasting ultrahigh frequency horizontally polarized Waves.

This invention makes use of a number of small loops shunted across a balanced transmission line arranged so that a large number of loops may be cophasally emergized thereby obtaining a large concentration of radiant power in a plane in which the radiation is distributed in a substantially circular pattern.

One object of this invention is a simple antenna with low windage that could be installed on tall metal masts and energized with one feeder.

Another object of our invention is an antenna that has power gain with respect to antennas of prior art so that two antennas of the invention could be used to produce the same concentration of radiated field as is normally produced by four or more antennas of the prior art. This result is particularly desirable where a large power gain is desired without employing the customary stacked earray consisting of a number of antennas requiring individual means for phasing in which the necessary fittings and transformers must be provided.

Still another object of our invention is an antenna which radiates horizontally polarized waves substantially equally in all horizontal directions when installed on a metal pole.

A further object of our invention is an antenna which is so arranged that it may be conveniently fed by a coaxial transmission line having a characteristic impedance of the order of 50 ohms.

These and other objects of this invention will make the invention more clearly understood with the aid of the figures in the drawing in which:

Figure 1 shows the elevation of a slotted ring antenna as viewed lengthwise of the antenna;

Figure 2 shows an enlarged section taken on the line 22 of Figure 1;

Figure 3 shows an enlarged view of one ring of Figure l on substantially the same scale as Figure 2;

Figure 4 shows a section taken on the line 4-4 of Figure 2; and

Figure 5 shows a Smith chart of reactances for slots of the present invention at opposite ends.

In the construction of Figure l, 1 is a coaxial transmission line feeder which is provided with two branches, 2 and 3, having both inner and outer conductors. Considering the branch 2, its inner conductor 4 emerges from the coaxial cable at 5 while the outer conductor 6 is fixed to a plate or disc 7 which makes contact with longitudinally extending channel element 8 and the rods 9 and It) on the outside of the ring flanges 11 and 12 (Figure 2) but not with the inner conductor 4-.

The antenna itself is built up as a series of substantially flat rings 13 in which the dimension at right angles to the plane of the ring is smaller than the dimension in the plane of the ring itself.

For example, a working model antenna built to A scale and, therefore, operating at a frequency of 720 me. has

rings made of .094 sheet conductive material having an outer diameter of 2.75" and an inner diameter of 1.75" and spaced 1.42" apart. The edges of the rings were not rounded so that the cross section of a portion of the ring was rectangular. The dimension in the plane of the ring was .5" while the dimension perpendicular thereto was .994. In practice it is desirable to make the cross section not rectangular but elliptical or diamond shaped so as to reduce the wind pressure further, while maintaining sufficient rigidity so that a man can climb along the rings using the rings as rungs of a ladder. This is particularly convenient when the antenna is designed for use at frequencies for V. H. F. television range.

The reason for making the perpendicular dimension as small as possible is that this dimension determines the pressure that wind will exert on the antenna and, therefore, to a large degree the overturning moment at the base of the supporting mast. Our experiments show that the thickness of the ring should be small compared to its diameter that the dimension of the cross section in the plane of the ring should be large, for example, 5 /3 times larger than the thickness of the ring in order to avoid an excessive number of rings per wave length in the antenna structure. With rings having a 5 /3 to l aspect ratio, for example, those mentioned in our example above, it is possible to space these rings .036 wave lengths apart whereas a smaller aspect ratio would result in a closer spacing of the rings anc, therefore, in the use of a far greater number per wave length for a given diameter ring.

The ring may have a fiat to somewhat oval cross section as shown in Figure 4- which is open at one side providing an air gap 14 (Figure 2). Each ring is provided with flat flanges ll and 12 at each side of the air gap perpendicular to the plane of the ring and extending longitudinally in the line of the succession of the rings. The rods 9 and to are secured to the outside of these fiat flanges lengthwise of the antenna and are adjustable by means of clamps or screws to the flanges so that their distance apart across the air gap may be adjusted. Slots Ill and 12' are provided across the flanges ill and 12 perpendicular to the air gap so that the rods 9 and li may be adjusted in position across the flanges by securing screws and bolts to ition the rods nearer together or further apart. The rin are widened out at the back side with a wide flat flange as shown at 15 forming a widened ring upon wh ch the longitudinal channel element 8 is secured.

The center conductor 4 emerges from the coaxiallinc insulated from the outer conductor and the disc 7 and extends through the center of the rings to a ring 13' to which a radial conductive strap 16 is fastened from the center conductor. This strap is adjustably connected from the inner conductor to the ring to provide the desired impedance. If it were at the back of the ring in the region of the channel 8, the connection would be a short circuit. As connected near the edge of the slot, the impedance may be of the order of 50 ohms to provide the desired impedance to match that of the coaxial feeder transmission line 2 and provide the same impedance substantially at the point where the inner and outer conductor separate.

A feature of the present invention is the single adjustment of the frequency of the antenna for the proper radio frequency and high frequency transmission. The antenna radiation should be such that the group of rings should span approximately an apparent half Wave length between the input end of the coaxial line and the ground connection at the end ring. In this way, all of the rings radiate substantially in phase to produce horizontal radiation and provide maximum horizontal radiation. The rods 9 and 10 are for this purpose located at selected distance apart on the flanges 11 and 12 and may be moved together or apart to adjust the velocity of propagation along the slot.

If the rods are moved together the velocity ofpropagation is increased and, therefore, the groups of frequencies are lowered and the wave length made longer.

The length-of the line=B in Figure 1 must be approximately an integral multiple of a=halfwave length at-the highfrequency radiated for optimum impedance matching to a 50 ohm coaxial feeder line. When the rods 9 and 10 are adjusted closer together the capacity across'the slot will be increased and, therefore, the velocity of propagation along the slot willbe decreased and ordinarily the antenna length to span an apparent half wave length would be decreased.

This is not too desirable, namely to shorten the antenna length. .It is far preferable to retain'the desired length for difierent broadcasting:frequencies and for this purpose it is better to vary the apparent velocity of propagation retaining the wave length constant and adjust the frequency with the apparent change in velocity of propagation.

In Figure there is plotted typical slot impedances. Curve a shows the slot impedance at the center of the slot from the inner conductor to the opposite side of the slot to which inner conductor is connected which ispractically the impedance of the slot at this point. The curve shows the plot over a changing frequency and indicates a variation from about 56 /2 ohms to 93 ohms covering a frequency change from 198 megacycles to-209 megacycles.

Curve [2 shows a plot of impedance at the input to the end seal plate namely, the plate 7 connected to the outer conductor. The impedance over the same frequency range is approximately 50 ohms varying in extreme limits from about 50 ohms to 60 ohms over the range. This is because of the construction and design of the line 4 and the impedance between it and the effective outer conductor made up of the rods, channel and shunting rings which curls up the impedance curve as indicated in Figure 6 holding the impedance practically constant over the frequency range.

Having now described my invention, I claim:

1. An antenna structure formed of a set of substantially flat coaxial rings having substantially larger dimensions in the plane of the ring than perpendicular thereto and having air gaps therein, conductive means securing said rings together at parallel spaced distances apart, a coaxial cable for feeding said antenna having an inner conductor extending coaxially through said rings and connected to one ring of the set remote from the first ring where the inner conductor first passes through the set of coaxial rings and having an outer conductor connected to said means.

2. An antenna structure comprising a longitudinally extending supporting conductive bar, a plurality of conductive rings having substantially larger dimensions in the plane of the ring than perpendicular thereto and spaced on and secured to said bar at right angles thereto having air gap openings diametrically opposite the position of said bar, and a coaxial cable for feeding said antenna having an inner conductor extending coaxially through said rings and connected to a ring of the plurality of rings remote from the first ring where the inner conductor first passes through the plurality of rings and having an outer conductor connected to said conductive bar near said first ring.

3. An antenna structure comprising a-longitudinally extending supporting conductive channel member, a plurality of conductive rings-having substantially larger dimensions in the plane of the ring than perpendicular thereto and spaced onand secured to the flat side of said channelmember at right angles thereto having said-conductive rings having air gapopenings diametrically opposite the position of said bar,"and a coaxial cable foifeeding said antenna having an inner conductor extending coaxially through-s'aid-rings and'bonnectd to a ring of the plurality of rings remote from the first ring where the 4 inner conductor firstv passes through the plurality-of rings and having an outer conductor connected to said channel member.

4. An antenna structure formed of a set of substantially flat coaxial rings having substantially larger dimensions in the plane of the ring than perpendicular thereto and with air gaps in each ring aligned along the antenna, conductive means securing said rings together at parallel spaced distances apart, a coaxial cable for feeding said antenna having an inner conductor extending coaxially through said rings and 'connected to one ring of the set remote from the-firstring where the inner-conductor first passes through'the set of coaxial rings and having an outer conductor connected to' said means adjacent first said ring.

5. An antenna structure comprising a conductive supporting bar, a plurality of conductive rings having substantially larger'dimensions in the plane ofthe-ring than perpendicular thereto 'and' each ring having an air gap opening. positioned diametrically opposite the supporting bar, a coaxi'al cablefeedingsaid rings with an internal conductorextendingcoaxially through said rings, an extern'alconductorconnectedto said supporting bar just before the point of ent'ryof said internal conductor through said rings and-aconducting strap from the internal conductor to one of said rings remote from thefirst ring through which-the inner' conductor passes.

6. An antenna structure comprising a conductive supporting bar, a plurality of conductive rings havingisubstantially-larger dimensions in the plane of the ring than perpendicular thereto and each ring having an air gap opening, said-opening extending in a direction on the structure parallel to said supporting bar, a coaxial-cable feeding to saidringshaving an internal conductor extending coaxially through therings and an external conductor connected-to said supporting bar in the-region before the-first ring anda conductive strap from the internal conductorto'oneof-the rings remote from the first'ring through which inner conductor passes.

7. An antenna structure comprising a conductive supporting bar, a plurality of conductive rings having substantially larger-dimensions in the plane of the ring than perpendicular thereto and each ring having an air gap opening, saidopening extending in a direction onthe structure parallel to said supporting bar, a coaxial cable feeding to the antenna structure between two adjacent conductive rings with branch coaxial cables extending in opposite directions through said rings, means connecting the external conductors to said conductive supporting bar before the cable extends through said rings, means extending the internal conductors in opposite irections through said rings and meansconnecting said internal conductors in opposite directions through said rings and means-connecting said internal conductors at a remote point to said rings.

- 8. An antenna structure comprising a conductive sup porting bar, a plurality of conductive rings having substantially larger dimensions in the plane of the ring than perpendicular thereto and each ring having an air gap opening, said opening extending in a direction on the structure parallel tosaid supporting bar, a coaxial cable feeding to the antenna structure between two adjacent conductive rings with branch coaxial cables extending in opposite directions through said rings, means connecting the external conductors to said conductivesupporting bar before the cable extends through said rings, means extending the internal conductors in opposite directions through said rings and means connecting said internal conductors at a remote point to said rings, and means for adjusting the impedance at the end connections to the rings to substantially the same value as the connection of the external conductor at the supporting bar.

9. An antenna structure comprising a conductive supporting bar, a plurality of conductive rings having substan'tially'larger dimensions in the'plane of the ring than perpendicular thereto and each ring having an air gap opening, said opening extending in a direction on the structure parallel to said supporting bar, a coaxial cable feeding to said rings having an internal conductor extending coaxially through the rings and an external conductor connected to said supporting bar in the region before the first ring, a rod secured along the structure on each side of the air gap for adjusting the apparent wave length thereof and a conductive strap connecting from the internal conductor to one of the rings remote from the first ring through which theinner conductor passes.

10. An antenna structure comprising a conductive supporting bar, a plurality of conductive rings having sub stantially larger dimensions in the plane of the ring than perpendicular thereto and each ring having an air gap,

opening, said opening extending in a direction on the structure parallel to said supporting bar, a coaxial cable feeding to said rings having an internal conductor extending coaxially through the rings and an external conductor connected to said supporting bar in the region before the first ring, a rod secured along the structure on each side of the air gap, means for adjustably positioning the rods to Widen and narrow the air gap for adjusting the apparent wave length thereof and a conductive strap connecting from the internal conductor to one of the rings remote from the first ring through which the inner conductor passes.

11. An antenna structure comprising a conductive supporting bar, a plurality of conductive rings having substantially larger dimensions in the plane of the ring than perpendicular thereto and each ring having an air gap opening, said opening extending in a direction on the structure parallel to said supporting bar, a coaxial cable feeding to said rings having an internal conductor extending coaxially through the rings and an external conductor connected to said supporting bar in the region before the first ring, said rings having a flat flange on either side of the air gap in the plane of the air gap, a rod secured to said fiat flanges on each side of said air gap for adjusting the apparent Wave length thereof and a conductive strap connecting from the internal conductor to one of the rings remote from the first ring through which the inner conductor passes.

12. An antenna structure formed of a set of substantially flat coaxial open rings having substantially larger dimensions in the plane of the ring than perpendicular thereto and providing an aligned air gap lengthwise of the structure, means attaching said rings diametrically opposite the air gap to a channel member being electrically conductive and extending lengthwise of the structure, a coaxial feeder having an inner conductor extend through the rings and electrically coupled to one of said rings and an outer conductor conductively connected to said channel member at a point before the inner conductor enters the first ring.

13. An antenna structure formed of a set of substan tially fiat coaxial open rings having substantially larger dimensions in the plane of the ring than perpendicular thereto and providing an aligned air gap lengthwise of the structure, means attaching said rings diametrically opposite the air gap to a channel member being electrically conductive and extending lengthwise of the structure, a coaxial feeder having an inner conductor extend through the rings and electrically coupled to one of said rings and an outer conductor conductively connected to said channel member at a point before the inner conductor enters the first ring comprising a conductive seal surrounding the outer conductor and connected to said channel member.

14. An antenna structure formed of a set of substantially flat coaxial open rings having substantially larger dimensions in the plane of the ring than perpendicular thereto and providing an aligned air gap lengthwise of the structure, means attaching said rings diametrically opposite the air gap to a channel member being electrically conductive and extending lengthwise of the structure, a coaxial feeder having an inner conductor extend through the rings to a point not substantially more than half way to the end ring and having at its end a conduc tive connection connected to a ring in the vicinity of the air gap and an outer conductor conductively connected to said channel member at a point before the inner conductor enters the first ring.

References Cited in the file of this patent UNITED STATES PATENTS 2,467,961 Caraway et a1. Apr. 19, 1949 

